This invention relates to an organosolv progress for the delignification of lignocellulosic material to produce a high yield of fibrillateable cellulose pulp having a low residual content of lignin while also obtaining reactive lignin as a by-product. In contrast to prior art processes, the method of this invention is directed to a high temperature process controlled so as to cause solvation of reactive lignin to predominate while suppressing degradation of cellulosic material in order to enable recovery of high quality cellulose pulp and of reactive lignin.
Numerous commercial processes exist for producing cellulose pulp from wood. Generally stated, those processes which rely on inorganic chemicals as the extracting agent do not yield lignin as a valuable by-product. The liquor used may take many different forms but the extraction process normally proceeds in an acidic or in a basic environment. If a high quality lignin by-product is to be recovered, the process used is usually of the so-called organosolv type, in which the liquor is an aqueous solution of an organic solvent. Exemplary of such a process is U.S. Pat. No. 3,585,104 which teaches the use of a digesting liquor containing an aqueous mixture of lower aliphatic alcohols such as methanol, ethanol, propanol or aqueous mixtures of the lower aliphatic ketones such as acetone, as appropriate digesting or pulping agents. As is conventional in the prior art with respect to such liquors, U.S. Pat. No. 3,585,104 teaches a pulping temperature of between 150.degree. C. to 200.degree. C. and common residence times of an hour or more at the cooking temperature. No consideration is given with respect to the overall cooking time during which the liquor is subjected to a temperature which will promote cellulose/hemicellulose degradation, i.e., the time during which the liquor and consequently the wood material therein is subjected to temperatures exceeding about 150.degree. C.
Also exemplary is U.S. Pat. No. 2,037,001 to Aronovsky wherein it is taught to employ an aqueous liquor containing a mono-hydroxy alcohol which has at least four carbon atoms of which at least three are in a straight chain. The invention disclosed in that patent is the use of an alcohol, as aforesaid, which is not miscible with water in all proportions at higher temperatures. Aronovsky teaches that when the aqueous liquor is at an elevated digesting temperature, the water and alcohol form a homogeneous solution in which the pulping takes place, but upon cooling to room temperature, the liquor forms two immiscible phases. One phase is the water phase containing inorganic chemicals in solution, and the other phase is the alcohol phase containing extracted lignin and other by-products. The pulp is in a solid phase and is separated from the liquid phase for recovery. It is taught that at the lower temperature at which the pulp is recovered, there is less opportunity for lignin to precipitate back onto the pulp because it remains solubilized in the alcohol layer which is stronger than when miscible with and hence diluted by water at the higher temperatures. It is taught that a purer lignin extract can be recovered from the alcohol layer and that more economical recovery of the alcohol is possible.
Aronovsky also teaches that extractive digestion of ligneous materials in an aqueous alcohol solution proceeds best in acidity. This patent recognizes that too much acidity exerts a hydrolytic action on the cellulose but that if alkalis such as caustic soda, sodium sulphide and sodium carbonate are added to avoid too low a pH, they exert an inhibiting action on the process of solvating lignin. In contrast, and in order to avoid this inhibitive action, urea in relatively small quantities is taught to be added as an agent to prevent high acidity in the digester. The result is that the digestion takes place at a higher pH than where urea is not used. The patent teaches the use of less than 5% of urea (yielding an acidic final pH) and that this amount of urea is fully as effective as 40% urea (which would yield a basic final pH).
Example 1 of the Aronovsky patent teaches a digestion temperature of 185.degree. C. and a cooking time of 2 hours at that temperature with no urea addition to yield 39.2% pulp with 2% lignin content, which pulp is noted to be comparable to that obtained by "the known `soda process`". In Example 2, the process of Example 1 was varied by displacing the cooking liquor and dissolved content with hot fresh liquor after one hour cooking time and continuing the cooking for an additional hour, yielding a higher percentage (51%) of pulp, residual lignin content being unspecified. Examples 3 and 4 of the patent employ the process but with different woods, teaching digestion temperature of 185.degree. C. and cooking time of 5 hours at that temperature, without urea addition, and yielding 45.7% pulp with 10% residual lignin and 50.5% pulp with 11.9% residual lignin respectively. Examples 5 and 6 use different alcohols, teaching digestion temperature of 175.degree. C. and a cookiing time of 4 hours at that temperature, without urea addition, to yield 53% and 54% pulp respectively, no residual lignin content being specified. Example 7 of the patent teaches the use of 4.5% urea based on dry wood at a digestion temperature of 175.degree. C. and cooking time of 4 hours at that temperature in order to achieve a high yield of cellulose pulp. A yield of 67.2% pulp is specified but residual lignin content thereof is not mentioned. In discussing Example 7 in comparison with the same process without urea, it is noted that "urea functions by decomposition to provide ammonia which is available to neutralize acid formed in the digestion, thus preventing a low pH (high acidity) in the digester. The result is that the digestion takes place at a higher pH than where urea is not used. This minimizes loss of and degradation and alteration of celluloses and pentosans of the original wood." These examples are in accord with the conventional pH, temperatures and times at cooking temperatures, i.e. an acidic environment, temperatures less than 200.degree. C. and long cooking times at the cooking temperature.